In recent years, various resonators using piezoelectric materials such as a film bulk acoustic wave resonator (FBAR) and a surface acoustic wave (SAW) resonator have come into practical use as small and high-performance resonators. Especially, the film bulk acoustic wave resonator has been recently attracting attention in view of uses where a high resonant frequency such as GHz band is required, because the resonant frequency thereof is easily increased due to its structure compared to the surface acoustic wave resonator.
As described in Japanese Patent Application Laid Open No. 10-270979, the film bulk acoustic wave resonator is basically constituted of an upper electrode, a lower electrode and a piezoelectric film interposed therebetween. A certain resonance characteristics can be obtained by applying a high frequency signal between the upper electrode and the lower electrode. The resonant frequency of the film bulk acoustic wave resonator mainly depends on film thicknesses of the upper electrode, the lower electrode and the piezoelectric film.
Film bulk acoustic wave resonators include the diaphragm type and the SMR type. In structure, a diaphragm type film bulk acoustic wave resonator has a substrate cavity below a resonant structure comprised of an upper electrode, a lower electrode and a piezoelectric film, with both sides (the upper electrode side and the lower electrode side) of the resonant structure being open to the air.
In contrast, in place of a substrate cavity, an SMR type film bulk acoustic wave resonator has an acoustic multilayer between the substrate and resonant structure comprised of a plurality of reflective films. The acoustic multilayer reduces undesired bulk wave reflections from the substrate. Because it does not require the formation of a substrate cavity, an SMR type film bulk acoustic wave resonator is relatively easy to fabricate, making it suitable for mass production, and also provides high reliability, since the mechanical strength is not reduced by the existence of a cavity.
However, SMR type film bulk acoustic wave resonators are known to be generally inferior to the diaphragm type concerning resonance characteristics. This is because the air interface on both sides of the resonant structure in the case of the diaphragm type enables relatively free resonance, while in the case of the SMR type, bulk waves propagated on the substrate side that are not adequately reflected by the acoustic multilayer reach the substrate and are reflected thereby, giving rise to spurious bulk waves.